Aminothiol supported dialdehyde cellulose for efficient and selective removal of Hg(II) from aquatic solutions

The increasingly serious problem of mercury pollution has caused wide concern, and exploring adsorbent materials with high adsorption capacity is a simple and effective approach to address this concern. In the recent study, dialdehyde cellulose (DAC), cyanoacetohydrazide (CAH), and carbon disulfide (CS2) are used as raw materials for the (DAC@CAH@SK2) preparation material through the three-steps method. By utilizing the following characterization techniques; thermogravimetric analysis (TGA), N2 adsorption–desorption isotherm (BET), elemental analysis, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD), 1HNMR and Energy Dispersive X-ray Spectroscopy (EDS) of DAC@CAH@SK2 composite. The point of zero charge (pHPZC) for the prepared DAC@CAH@SK2 also was examined. From the batch experiments, the optimum conditions were found to be pH (5–8), an Hg2+ concentration of 150 mg/L, a DAC@CAH@SK2 dose of 0.01 g, and a contact time of 180 min with a maximum adsorption quantity of 139.6 mg/g. The process of Hg2+ adsorption on the DAC@CAH@SK2 material was spontaneous exothermic, monolayer chemisorption, and well-fitted to Langmuir and pseudo-2nd-order models. The DAC@CAH@SK2 selectivity towards the Hg2+ was examined by investigating the interfering metal ions effect. The DAC@CAH@SK2 was successfully applied for the Hg2+ removal from synthetic effluents and real wastewater samples with a recovery % exceeding 95%. The prepared DAC@CAH@SK2 was regenerated using a mixture of EDTA and thiourea. Also, FT-IR analysis indicates that the synergistic complexation of N and S atoms on DAC@CAH@SK2 with Hg(II) is an essential factor leading to the high adsorption capacity.


Preparations
preparation of dialdehyde cellulose (DAC) and the aldehyde content (%) estimation In the complete absence of light, the cellulose (one gram) oxidation occurred using 4% KIO 4 (100 ml), as present in Fig. 1.The previous mixture was shaken for 6 h.The obtained oxidized cellulose (DAC) was washed by dist.H 2 O followed by the DAC drying step in an oven at 50 °C39 .The aldehyde content (AC %) of the prepared DAC was determined as reported previously 39,40 , 0.1 g of the prepared DAC was added to 25 mL of 250 mM pH 4 adjusted hydroxylamine HCl then the mixture was stirred at room temperature in complete darkness for 2.5 h.Then, DAC was filtered and dried in an oven for 60 min at 70 °C.The filtrate was back-titrated utilizing 0.1 M NaOH to pH 4 and the reaction endpoint was achieved when the color shifted from red to yellow.The control experiment was performed by replacing the DAC with native material.The estimation of the prepared DAC aldehyde content percentage (AC (%)) has occurred using Eq.(1).
Where V sample and V control are the volumes of NaOH in the case of oxidized cellulose and native cellulose powder, respectively.While m is sample weight and Mwt is cellulose molecular weight.
Preparation of cyanoacetohydrazide/carbondisulfide modified dialdehyde cellulose (DAC@CAH@SK 2 ) sorbent At first, 4 g of cyanoacetohydrazide were dissolved in ethanol and were added to 1.5 g of DAC with the addition of 2 drops of glacial acetic acid.The previous mixture was allowed to reflux for 6 h at 70 °C to form DAC@ CAH material.Then the mixture was filtered and washed with ethanol.After drying the DAC@CAH material at 45 °C, it was added to 10ml CS 2 then 40 ml of ethanol was added.The mixture was stirred for 8 h at room temperature.Then 1ml of KOH solution was added to the previous mixture and allowed to stir for 15 min at room temperature.Finally, the mixture was filtered and the obtained DAC@CAH@SK 2 material was washed with ethanol and dist.H 2 O and dried in an oven at 45 ο C. The subsequent steps of preparation of DAC@CAH@ SK 2 are illustrated in Fig. 2.  www.nature.com/scientificreports/
C i and C f are the initial and equilibrium Hg 2+ concentration (ppm), respectively.While m (g) is the DAC@ CAH@SK 2 dose and V (L) is the adsorbate solution volume.

Adsorption Selectivity
A total of 10 mg of DAC@CAH@SK 2 composite was added to a 10 ml solution of binary metal ions and multiple metal ions synthetic mixtures (with the same concentrations of each metal ion (150 mg L −1 )) and shaken at 25 °C for 2 h.The added metal ions concentrations were estimated by ICP OES and the adsorption capacities were calculated with Eq. ( 3), and then the adsorption selective coefficient (α) was defined as presented in Eq. ( 4): Application 150 mg/L of Hg 2+ was spiked to the real (sea, waste, and tap) water samples.Before the spiking step, the real water samples were digested by adding a mix of K 2 S 2 O 8 (0.5 g) and H 2 SO 4 (5ml, 98% (w/w)) to 1000 ml of each water sample and then heated for 120 min at 90 °C for complete digestion of presented organic materials.After cooling to room temperature, 0.01 g of DAC@CAH@SK 2 was added to the prepared samples, and the pH value was adjusted to 7 with continuous shaking for 180 min.The solutions were centrifuged and again another 0.01 g of DAC@CAH@SK 2 was added to the supernatant to ensure the complete separation of analytes.The remaining Hg 2+ was determined using ICP OES.

DAC synthesis
The KIO 4 oxidizing agent is a known selective one that is utilized for the oxidation of two OH groups that are present on the glucopyranoside ring's C 2 -C 3 neighboring carbon atoms.KIO 4 cleaves the bond between C 2 -C 3 and the OH groups are converted into two dialdehyde groups.The oxidation degree which represents the percentage of monosaccharide units that reacted with KIO 4 is calculated by aldehyde content determination 39 .The AC % of the synthesized DAC is 38.93% as shown in Table 1.
The adsorption capacity of Hg (II) on DAC@CAH@SK2 The adsorption capacity of coexisting metal ion on DAC@CAH@SK2 www.nature.com/scientificreports/Physicochemical properties of native cellulose, DAC@CAH, and DAC@CAH@SK 2 composite Specific surface area is an essential parameter, as it has a great effect on adsorbent capacity toward pollutants.The surface area of DAC, DAC@CAH, and DAC@CAH@SK 2 was measured and is presented in Table 2.The surface area was decreased by modification of cellulose with cyanoacetohydrazide to 8.137 m 2 /g.This decrease may be due to the reaction of cyanoacetohydrazide with the cellulose.While the surface area increased after the reaction with CS 2 to 268.729 m 2 /g, which may be returned to the grafting of SH groups into the material.The solubility of DAC@CAH@SK 2 was studied by utilizing various solvents like sodium hydroxide (0.1-1 M), HCl (0.1-1 M), and ethanol 99.9%.It was noticed that DAC@CAH@SK 2 is not soluble in any of the utilized solvents.

Characterization
Elemental analysis Table 3 illustrates the EA results of native cellulose, DAC@CAH, and DAC@CAH@SK 2 composite.The results obtained indicate that the nitrogen content significantly increased to 11.73% after the modification that occurred through the cellulose oxidation followed by condensation with cyanoacto hydrazide.Nitrogen content decreased to 8.43% with increasing sulfur content to 12.86% after the reaction with Carbon disulfide.These results confirm that DAC-CAH and DAC@CAH@SK 2 are successfully formed.The concentration of the inserted cyanoacetohydrazide units was calculated to be approximately 1.557 mmol g −1 .

Scanning electron microscope (SEM)
Surface morphologies of DAC, DAC@CAH, DAC@CAH@SK 2 , and DAC@CAH@SK 2 -Hg 2+ are shown in Fig. 3a-d, respectively.Figure 3a and b showed that DAC@CAH has more roughness than DAC material which may be attributed to the chemical modification by cyano-aceto hydrazide.DAC@CAH@SK 2 has more roughness and porous surface than DAC@CAH as shown in Fig. 3c, which may be attributed to the modification of DAC@CAH with CS 2 in the presence of KOH.As shown in Fig. 3d, the SEM of DAC@CAH@SK 2 -Hg 2+ shows a brighter surface than the other materials because mercury has better electric conductivity than the modified cellulose materials 50,51 .
FTIR spectra FTIR spectra of native cellulose, its derivatives (DAC, DAC@CAH, and DAC@CAH@SK 2 composite), and DAC@ CAH@SK 2 -Hg 2+ are represented in Fig. 4. Native cellulose (Fig. 4a) showed a number of distinctive peaks as the C-O stretching vibrations that appeared in the range of 1000-1200 cm −1 .While those in the range of 1260-1410 cm -1 and 3200-3600 cm −1 are assigned to OH bending and stretching vibrations, respectively.Moreover, the peaks of C-H stretching vibrations that are present between 2700 and 3000 cm −139 .FT-IR spectrum of DAC is present in Fig. 4b and exhibits a new peak at about 1730 cm −1 which may be assigned to the C=O stretching vibrations of the aldehyde group (RCHO) 40 .DAC Modification with Cyanoactohydrazide results in some shifts in the IR spectrum of the DAC@CAH, an observed peak at about 1680 cm −1 , which may be returned to C=N formation between the DAC' aldehyde groups present and amino groups of the added Cyanoactohydrazide as present in Fig. 4c and 23 .Moreover, the peak between 3600 and 3100 cm −1 became broader due to the OH and amino groups' absorption peaks overlapping 52 .The spectrum of the DAC@CAH@SK 2 composite, Fig. 4d, shows the presence of cyano group stretching vibrations at about 2174 cm -1 that may be appeared after the reaction with CS 2 because of the tautomerism phenomenon 53 .Furthermore, due to the presence of overlapping bands at the range of 500-600 cm −1 , the C-S peak was not visible individually 54 .The second derivative FT-IR was obtained in the 1300-500 cm −1 range to confirm the presence of C-S characteristic peak.As presented in Fig. 5 for DAC@CAH@SK 2 through the 2nd derivative, a band at 590 cm −1 attributed to C-S stretching vibrations was detected 55,56 .

H-NMR
Besides the XRD technique, both liquid and solid phases of NMR are used to study the cellulose structure.Also, the solid phase of 13 C NMR is used to investigate the various cellulose polymorphs but it is less available and requires more extensive investigations than liquid phase NMR 40,57,58 .In all molecular solvents, both DAC and DAC@CAH@SK 2 cellulosic materials are completely insoluble.The 1 H NMR of DAC and DAC@CAH@SK 2 were investigated by utilizing the DMSO/Trifluoroacetic acid mixture 58 .Figure 6 represents the 1 H NMR of DAC and DAC@CAH@SK 2 .The 1 H NMR of DAC present in Fig. 6a showed a peak at 2.03 ppm related to the proton that is present on C 2 or C 3 of the DAC.Broad peaks that appeared at 4 ppm and 4.96 ppm are related to C 1 proton and OH, respectively.Figure 6b represents the 1 H NMR of DAC@CAH@SK 2 that showed a new prominent peak, broad signal near 9.28 ppm that could be is related to protons of NH of the cyanoacetohydrazide 40,59 .

Thermal analysis (TGA)
Thermogravimetric analyses for the DAC@CAH@SK 2 before and after mercury ions adsorption were carried out in the temperature range of 30-900 °C to give information about materials thermal stability, Fig. 7 The thermograms demonstrated that each compound undergoes a series of different degradation steps.The thermal degradation of DAC@CAH@SK 2 and DAC@CAH@SK 2 -Hg 2+ were studied under the same conditions and the total residues were determined as following 27.89% and 21.15%, respectively.The lower value of total residues of DAC@CAH@SK 2 -Hg 2+ compared to the parent one (DAC@CAH@SK 2 ) suggests that mercury ions show a noncatalytic degradation effect during their complexation with the modified cellulose material 60 .

X-ray diffraction
The diffractograms of the untreated cellulose, DAC, and DAC@CAH@SK 2 are presented in Fig. 8. Key diffraction peaks of the cellulose were shown in the XRD patterns at roughly 15.178º, 16.59º, 20.5º, 22.835º, 28.05º, and 34.475º, with major intensities at peak values of 15.178º, 16.59º, 22.835º, and 34.475º.The primary peaks associated with crystallographic planes were (11̅ 0), ( 110), (200), and (004), respectively and these peaks were discovered to be related to cellulose type I [61][62][63] .The positions of these investigated peaks stay similar in DAC and DAC@CAH@SK 2 materials with some slight shifting in addition to the samples' crystallinity indices (CrI) were calculated according to the Segal method (Eq.5) and found to be 73.19,71.77, and 69.04% for untreated cellulose, DAC, DAC@CAH@SK 2 .This may indicate that the crystallinity of the functionalized samples (DAC and DAC@ CAH@SK 2 ) was slightly affected by the adopted chemical modification 40,64 .The crystallinity decrease is returned to the glucopyranose ring opening and cellulose backbone destruction that result from cellulose oxidation by the KPI and 2,3 dialdehyde cellulose (DAC) formation 65,66 .The (CrI) value of DAC@CAH@SK 2 is lower than that of DAC which indicates that the DAC@CAH@SK 2 is more amorphous than DAC.This could be illustrated according to their chemical structure as the DAC@CAH@SK 2 has a more compact structure than the DAC 40,64 . (

Point of zero charge (pH PZC )
In order to understand the Hg(II) adsorption mechanism by the prepared composite, the point of zero charge of DAC@CAH@SK 2 composite was studied as presented in Fig. 9.This investigation was obtained by measuring the pH at the point of zero charge (pH PZC ).Commonly, the chelating agent will show greater affinities for cations at a pH value higher than the value of its pH PZC and vice versa.The pH PZC value obtained for the DAC@CAH@SK 2 composite was approximately 6.85.Hg(II) adsorption by the DAC@CAH@SK 2 was expected to be enhanced at a pH value higher than the pH PZC value.

Effect of pH
The pH is considered an essential parameter as it can influence the solubility and the ionization degree of the studied chelating agent.Also, it can affect metal ion speciation.The adsorption behavior of DAC@CAH@SK 2 chelating agent has been investigated at the pH range of 1-10, which was selected to avoid metal precipitation that occurs in the very alkaline medium.The experiment was studied using 0.01 g of DAC@CAH@SK 2 composite immersed in 10 mL of 150 ppm Hg(II) solution for 180 min.As shown in Fig. 10, the Hg 2+ adsorption increased with pH increasing from 1 to 5 and became constant at pH range 5-8 then it decreased at pH higher than 8.
The prevailing species for the divalent mercury At pH 2 are HgCl 2 , HgCl + , Hg 2+ , and HgOHCl by percent % 63.25, 25.20, 3.9, and 2, respectively; in addition to the presence of HgOH + and Hg(OH) 2 in very minor quantities.While the prevailing species at pH 4 are 39.90% Hg(OH) 2 , 25.20% HgOHCl, and 10.02% HgCl 2 besides the other species that are present in very small percent such as HgOH + , HgCl + , and Hg 2+ .The Hg(OH) 2 and HgOHCl species are the prevalent ones at the pH range from 6 to 8 and present by percent 79.62% and 10.02%, respectively [67][68][69] .The higher affinity of the DAC@CAH@SK 2 composite to Hg(II) at higher pH is that the prepared composite contains SK groups that consider soft bases while Hg(II) ions are soft acids.In accordance with the  www.nature.com/scientificreports/with a pH pzc value equal to 6.5 in the pH range of 3-11.They indicated that the divalent mercury adsorption increased by pH value increasing from 3 to 8 and then decreased sharply as the pH value increased to 11.These investigations recommend that Hg(II) adsorption to cellulose-based adsorbents relies on the utilized adsorbent/ chelating agent properties such as pH pzc , functional groups, etc. in addition to the speciation of the divalent mercury in the studied solution.

Effect of chelating agent dose
The influence of DAC@CAH@SK 2 composite dose on Hg 2+ adsorption capacity (mg/g) and removal efficiency (%) was obtained by utilizing various doses of it and the results are present in Fig. 11.The Hg 2+ removal % increased rapidly as the chelating agent dosage increases from 0.001 to 0.01 g, with an increase in the adsorption capacity from 110 to 139.5 mg/g.While the removal % slightly increased from 93 to 97% by increasing the DAC@ CAH@SK 2 composite dose from 0.01 to 0.015 g this was accompanied by a decrease in adsorption capacity from 139.5 to 97 mg/g.This may be returned to the increase in DAC@CAH@SK 2 dose results in the specific surface area increase which means more available adsorption sites.

Effect of initial concentration of Hg 2+
To investigate the effect of initial concentration on Hg 2+ adsorption capacity, a 10 ml solution of Hg 2+ at a fixeddose of DAC@CAH@SK 2 chelating agent 0.01 g for Hg 2+ was taken at pH 6 for 3 h in range (25-400 ppm).After that, initial concentrations were varied and the corresponding capacities and removal percentages were obtained as shown in Fig. 12.It was noticed that the DAC@CAH@SK 2 adsorption capacity for Hg 2+ increased from 24.88 www.nature.com/scientificreports/ to 139.1 mg/g with Hg 2+ initial concentration increasing from 25 to 150 ppm.Moreover, with the increase of Hg 2+ initial concentration from 150 to 400 ppm, the DAC@CAH@SK 2 composite tends to stabilize.

Adsorption isotherms
The adsorption isotherm can be defined as follows; the studied adsorbate concentration relation with the adsorbed pollutant amount (q e ) at the adsorbent (chelating agent) surface at a fixed temperature.Dubinin-Radushkevich (D-R), Langmuir, and Freundlich's isothermal models, which can indicate the maximum adsorption capacity (q e ) and binding affinity, were applied in the linear form and the parameters were determined as present in Eqs. ( 6), ( 7) and ( 8), respectively.The dimensionless equilibrium factor (R L ) presented in Eq. ( 9), is an important parameter that is used in adsorbent-sorbate affinity prediction.Its values are explained as follows: if the R L value is found to be greater than 1.0 this means that the investigated material is unsuitable and unfavorable, while if it is found to be (0 < R L < 1), (R L = 0), or (R L = 1) this means the reaction is favorable, irreversible, or linear, respectively 23,35 .The Dubinin-Radushkevich model studies adsorption energetically and creates the assumption that the adsorption process relates to pore volume and surface porosity.Equation (10) represents the E DR which can be defined as following the adsorption mean free energy obtained from the D-R model.It reveals whether the adsorption process is chemical (8 < EDR < 16 kJ mol −1 ) or physical (E DR is lower than 8 kJ mol −1 ) 40 .
where C e (mg/L), 1/n, K L (L/mg), K F (mg/g), and K are the Hg 2+ concentration at equilibrium, the heterogeneity factor, Langmuir, Freundlich, and the Dubinin-Radushkevich constants, respectively.While, q e and q m , which are expressed in mg/g, are the Hg 2+ capacity at equilibrium and adsorption maximum amount.R which its value equals 8.314 J/mol and T which is expressed in Kelvin are the gas constant and the temperature, respectively.ε is the adsorption potential and is presented in Eq. (11).
The Langmuir, Freundlich, and D-R isotherms determined for the Hg(II) adsorption utilizing DAC@CAH@ SK 2 chelating agent are presented in Fig. 13 and their derived parameters (K L , K f , K, n, and q m ) are given in Table 4.The Hg(II) adsorption process using DAC@CAH@SK 2 composite follows the Langmuir isotherm model as the R 2 value for the Langmuir isotherm model is higher than that of Freundlich as presented in Table 4.This explains that the binding of Hg(II) ions to the active sites of the DAC@CAH@SK 2 composite is a chemisorption (6)  ln q e = ln q m − kε 2 (7) .Effect of Hg 2+ initial concentration (conditions: 0.01g of DAC@CAH@SK 2 was taken at pH 6 for 3 h in range 25 ppm-400 ppm of Hg 2+ ).
process (mono-layer).The R L value was calculated and found to lie between 0 and 1 as shown in Table 4 which implies that the Hg(II) adsorption by DAC@CAH@SK 2 composite is a favorable process that proves the applicability of the prepared composite for Hg(II) remediation from solutions 40 .The current adsorption process' E DR was calculated from the D-R model as presented in Eq. ( 10) and found to be 9.847 lies in the range of 8-16 kJ mol −1 which reveals that the Hg(II) adsorption process onto the DAC@CAH@SK 2 composite was chemisorption 40 .www.nature.com/scientificreports/

Effect of oscillation time and adsorption kinetics
In order to investigate the Hg(II) adsorption mechanism using the DAC@CAH@SK 2 material, the kinetic studies have occurred by examining the influence of oscillating time at various times from 30 to 300 min using 0.01 g of DAC@CAH@SK 2 which was added to a series of glass bottles contain 10 ml of 150 ppm Hg(II).Figure 14a illustrates the adsorption capacity of DAC@CAH@SK 2 at different oscillating times.It is noticed that the adsorption capacity of DAC@CAH@SK 2 increased with the increase of oscillating time from 30 to 180 min to reach q e value 139.5 mg/g.At an oscillating time of 180 min, the DAC@CAH@SK 2 adsorption capacity became constant and the Hg(II) adsorp tion attained equilibrium.
In order to determine the Hg 2+ adsorption rate-limiting step, kinetic investigations were carried out using three typical models; pseudo-1st-order, pseudo-2nd-order and the intraparticle diffusion model (IPD) model which are shown in Eqs. ( 12), ( 13) and ( 14), respectively.
The adsorption efficiency for Hg 2+ at equilibrium and at a certain time t (min) are expressed as q e (mg/g) and q t (mg/g), respectively.As well as K 1 , K 2 , and K diff are pseudo-1st-order, pseudo-2nd-order, and IPD constants, respectively.The C which is defined as the IPD equation intercept, was utilized in order to investigate the impact of the boundary layer.It was discovered that as the intercept value increased, the contribution of the rate-limiting step got higher.
Figure 14b-d presents the experimental data fitting to Pseudo-1st-order (PFO), Pseudo-2nd-order (PSO), and Intraparticle-diffusion (IPD) kinetic models.Moreover, the kinetic parameters (K 1 , K 2 , K diff , q e 1ads, q e 2 ads, and R 2 ) derived from the three models are shown in Table 5.The adsorption of Hg(II) by DAC@CAH@SK 2 achieved equilibrium within 3 h (Fig. 14a).When the correlation coefficients, R 2 , of both PFO and PSO models were compared, it was discovered that the results fit the PSO kinetic model better.It was noticed that two-line components appeared rather than a single one passing through the origin in the IPD model graph (Fig. 14d), which indicates that Hg 2+ adsorption onto DAC@CAH@SK 2 composite includes different diffusion stages that take place on and inside the DAC@CAH@SK 2 surface.In this case, it was demonstrated that it is not possible to describe the adsorption with one kinetic model.The IPD model for Hg 2+ adsorption by the DAC@CAH@SK 2 composite indicates that the adsorption provides various diffusion stages that occurred on the DAC@CAH@SK 2 composite surface and inside its surface.In the beginning, numerous active sites were available so the adsorption occurred quickly.Then, with oscillating time passing the DAC@CAH@SK 2 composite active sites decreased and the diffusion of Hg 2+ into pores becomes more difficult so the adsorption becomes slower 40,72,73 .

Thermodynamics
To study the nature of the Hg 2+ adsorption process onto the DAC@CAH@SK 2 surface in terms of spontaneity and feasibility and to estimate the degree of randomness at the solid/liquid interface, adsorption thermodynamic parameters ( G o ads , H o ads , and ( S o ads )) were determined at a temperature range (of 25-45 °C).Where G o ads , H o ads , and ( S o ads ) are Free energy, the heat of enthalpy, and adsorption entropy, respectively.Hg 2+ adsorption by DAC@CAH@SK 2 material was determined G o ads parameter was calculated from the following equations Eqs. ( 15), (16), and (17).The plotting of ln KC vs (1/T) temperature in Kelvin for the Hg 2+ adsorption onto the DAC@CAH@SK 2 composite is presented in Fig. 15.
As Kc, Cad, and Ce are a thermodynamic equilibrium constant, the Hg2+ concentration taken by DAC@ CAH@SK2 material at equilibrium (mg/g), and the Hg2+ concentration at equilibrium (mg/L), respectively.While R is the universal gas constant.
As presented in Table 6, the negative values of G o adsn and H o ads demonstrate that Hg 2+ adsorption by DAC@ CAH@SK 2 is spontaneous and exothermic, respectively.While the arrangement increasing and disorder lowering occurrence was proved from the negative value of ΔS o ads .

Effect of interfering ions and adsorption selectivity
The selectivity parameter is very essential to evaluate the DAC@CAH@SK 2 composite's adsorption properties 74,75 .Hence, The DAC@CAH@SK 2 adsorption selectivity for Hg(II) in the presence of different coexisting metal ( 12) t + 1 q e(ads) (13) t + 1 q e(ads) t www.nature.com/scientificreports/ions was carried out as shown in Table 7.At the Hg(II) adsorption optimum conditions, the DAC@CAH@SK 2 composite exhibits excellent selectivity for the Hg(II) ions with the interfering metal ions (Ni(II), Zn(II), Pb(II), Cu(II), and Cd(II)) either in binary systems or multiple-components synthetic mixtures.DAC@CAH@SK 2 exhibits selective adsorption recovery (Re, %) for Hg(II) of 100% with the interference of Cu(II), Ni(II), and Zn(II) ions.While DAC@CAH@SK 2 shows recovery higher than 97% in the presence of Cd(II) and Pb(II) ions.This excellent adsorption selectivity of DAC@CAH@SK 2 for Hg(II) is attributed to the presence of abundant nitrogen and sulfur-containing groups such as -NH 2 , -NH -and -SK on the surface of DAC@CAH@SK 2 , which exhibit strong affinity to Hg(II).When Pb(II) coexists with Hg(II), the selectivity coefficient is quite low, which may be illustrated by the hard-soft acid-base theory (HSAB).Both Hg(II) and Pb(II) are sorted as soft ions which have a strong affinity with thiol and nitrogen active groups that present on the DAC@CAH@SK 2 composite.DAC@CAH@SK 2 demonstrates outstanding recovery (Re, %) for Hg(II) of 102% and 104% in the presence of the following synthetic mixtures (Hg(II), Pb(II), Cd(II), and Zn(II)) and (Hg(II), Pb(II), Cd(II), Zn(II), and Cu(II)), respectively.As a whole, the DAC@CAH@SK 2 composite can potentially adsorb and separate Hg(II) in binary and multiple-metal ions systems.Metal sulfides, dithiocarbamates/thiocarbamates, thiosemicarbazones, thioureas, thiadiazole, and thiazoles are a number of agents that have been well identified for the materials' functionalization for Hg(II) adsorption/uptake.Carbon and sulfur groups are the major components of these agents.Because sulfur is very selective towards mercury, its usage in material modification/functionalization is particularly desirable.Furthermore, Hg(II) forms stable complexes with ligands containing nitrogen and oxygen active groups 76 .

Influence of ionic strength
The parameter of ionic strength was studied by utilizing Cl -, I -, and NO 3 -inorganic electrolytes in the form NaCl, KI, and NaNO 3 , respectively.It was investigated by utilizing 0.01 g of DAC@CAH@SK 2 that was added to 10 ml aqueous solution of 100 ppm of Hg(II), at 25 °C for 180 min and investigated electrolytes concentration Table 5. Kinetic parameters for the adsorption of Hg 2+ by DAC@CAH@SK 2 .
The DAC@CAH@SK 2 re-using was studied for five sorption-desorption cycles at the optimum conditions with sorption efficiency higher than 89% as shown in Table 8.It was predicted that DAC@CAH@SK 2 material could be a good sorbent for Hg 2+ removal from aqueous solutions.www.nature.com/scientificreports/

Applications
The application experiments of the DAC@CAH@SK 2 were obtained by adsorption of Hg 2+ (100 mg/L and 150 mg/L) from tap water, seawater, and wastewater samples to evaluate prepared chelating agent applicability in real samples.As displayed in Table 9, the Hg 2+ recoveries (%) from the tested real wastewater samples exceeded 95%.It was demonstrated that DAC@CAH@SK 2 has remarkable recoveries for the Hg 2+ that was spiked in the tested water samples proving that the DAC@CAH@SK 2 can be used for mercury removal from the aqueous environment in actual practice.

Plausible adsorption mechanism
To investigate the possible mechanism of Hg 2+ adsorption on DAC@CAH@SK 2 , SEM, digital images, and FTIR of the DAC@CAH@SK 2 and Hg 2+ -DAC@CAH@SK 2 were evaluated.
Digital photographs.The digital photographs of native cellulose, DAC, DAC@CAH, DAC@CAH@SK 2 , and Hg 2+ -DAC@CAH@SK 2 were shown in Fig. 17a-e, respectively.An obvious color changes after each step, it converted from the white color of the DAC to the pale sandy fawn color after modification with cyano-aceto hydrazide (Fig. 17b and c) and to dark yellow after modification with Carbon disulfide and potassium hydroxide as in Fig. 17d.The color of the modified cellulose changed into greenish yellow after the adsorption of Hg 2+ as in Fig. 17e.These changes indicated that the tendency of the modified cellulose towards the adsorption of Hg 2+ .
FTIR spectra.In Fig. 18, the DAC@CAH@SK2 composite spectrum before and after Hg 2+ adsorption is shown 54 .This showed that after Hg 2+ adsorption the cyano group peak disappeared and C-S between 500 and 600 cm −1 became broader than those of DAC@CAH@SK 2

54
. The azomethane characteristic peak which appears at 1650 cm −1 as presented in Fig. 18a showed an obvious shift after the complexation with the Hg 2+ ions to a lower value of 1632 cm −1 as shown in Fig. 18b.Additionally, the bands corresponding to -NH in the DAC@CAH@ SK 2 chelating agent were shifted in the spectrum of Hg 2+ -DAC@CAH@SK 2 .All previous indications led us to estimate that the complexation may be performed by forming a six-ring complex either by the lone pair of the cyano group's nitrogen atom and lone pair of the carbonyl group or through the -NH and a sulfur group of the C-S.The Hg 2+ complexation with the DAC@CAH@SK 2 may also carried out through the carbonyl lone pair and the -NH group results in five ring complex formation.Table 8.Repeated adsorption-desorption cycles for DAC@CAH@SK 2 regeneration by using 0.1 M thiourea/0.1 M HNO 3 mixture (1:1).www.nature.com/scientificreports/Energy-dispersive X-ray spectroscopy (EDS) analysis.The Hg(II) adsorption on the prepared chelating agent surface was confirmed and proved through the energy dispersive X-ray (EDS) analysis investigation as presented in (Fig. 19).The appearance of the Hg(II) characteristic peak in the range 1-3 keV demonstrates the Hg(II) adsorption on the DAC@CAH@SK 2 surface 77 .
Besides the previous evidences, SEM analysis indicates the Hg 2+ adsorption by the DAC@CAH@SK 2 composite as the surface of the DAC@CAH@SK 2 -Hg 2+ is brighter than that of DAC@CAH@SK 2 as shown in Fig. 3c  and d which returns to the mercury electric conductivity properties which result in DAC@SAH@SK 2 -Hg 2+ has better electrical conductivity than the DAC@SAH@SK 2 composite.
The mechanism of any metal ion adsorption onto a material usually is either a chemical reaction or an ion exchange reaction between the chelating agent active groups and the metal ions.Mechanisms applied in Hg(II) adsorption are ion exchange, precipitation, surface-complexation, and complexation/chelation.For thiol (-SH) functionalized adsorbents, the empty orbital of the Hg(II) can bond with free SH lone pair of electrons.Considering the analysis mentioned above, the possible mechanism of Hg(II) ions adsorption onto DAC@CAH@SK 2 composite could be as shown in Fig. 20.The Hg(II) ions complexes were formed via the coordination bonds with -Sk, C=O, C=N, and lone pair of N-H as presented in Fig. 20 forming five-membered ring and six-membered ring stable complexes 76,78 .
Performance of the prepared DAC@CAH@SK 2 A comparison between DAC@CAH@SK 2 chelating agent' q max (maximum adsorption capacity) for Hg 2+ with other adsorbents that were previously used for Hg 2+ pollutant removal was obtained to improve DAC@CAH@ SK 2 value as shown in Table 10.We can observe that the Hg 2+ adsorption by the DAC@CAH@SK 2 is respectably positioned to other researches with a q max of 139.6 at 25 °C since presents a q max value higher than the other reported adsorbents' q max .Because of environmental concerns and development requirements, the desorption process and adsorbent regeneration is an essential matter for any adsorbent to evaluate the adsorbent reusing for industrial applications.

Conclusion
We demonstrated the fabrication of the DAC@CAH@SK 2 composite and its capability to adsorb Hg(II) metal ions from aqueous solutions.The prepared material was fabricated by oxidation cellulose and subsequent chemical modification processes.The prepared materials (DAC, DAC@CAH, DAC@CAH@SK 2 , and DAC@CAH@ SK 2 -Hg 2+ ) were characterized by different techniques such as SEM, TGA, XRD, FT-IR, CHNS, and BET.The DAC@CAH@SK 2 exhibited effective and rapid adsorption behavior toward Hg(II) in aqueous solutions, where the optimal initial pH was 7. The adsorption mechanism was revealed by observing the adsorption isotherm and adsorption kinetics.The equilibrated adsorption capacity as a function of the Hg(II) concentration was described precisely using the Langmuir isotherm model, strongly suggesting that the adsorbates form a monolayer with a homogenously distributed surface adsorption energy on the DAC@CAH@SK 2 surface.The maximal adsorption capacity obtained from the model was 139.6 mg•g −1 for Hg(II) ions.Kinetic studies with a pseudo-2 nd -order kinetic model clearly showed the chemisorption of a single doubly charged mercury ion onto the DAC@CAH@ SK 2 composite.In addition to The negative free enthalpy (ΔG o ) and enthalpy (ΔH o ) values indicated that the adsorption of Hg(II) onto DAC@CAH@SK 2 is spontaneous and exothermic over the investigated temperatures range.The prepared DAC@CAH@SK 2 composite can be utilized effectively for the selective adsorption and  recovery of Hg(II) ions from aqueous solutions.The comparison of q e of DAC@CAH@SK 2 composite with other sorbents used in Hg(II) removal was reported in the literature.The DAC@CAH@SK 2 material was applied for the removal of Hg(II) from real waste samples.These results highlight the impact of the chemical surface functionalization of cellulose via oxidation followed by chemical modification toward water remediation.The DAC@CAH@SK 2 Synthesis and its use for Hg 2+ adsorption is shown in Fig. 21.

Figure 15 .
Figure 15.Plot of ln K C vs (1/T) absolute temperature for the adsorption of Hg 2+

Table 2 .
Surface area of the native cellulose and the prepared materials.

Table 3 .
Elemental analysis of native cellulose and the prepared compounds.

Table 6 .
Thermodynamic parameters for the adsorption of Hg 2+ onto DAC@CAH@SK 2

Table 7 .
Effect of interfering ions on the recovery of Hg(II) by DAC@CAH@SK 2 sorbent.Effect of ionic strength on the Hg(II) adsorption (conditions: 0.01g of DAC@CAH@SK 2 was taken at 10 ml 150 ppm of Hg 2+ at pH 6 for 3 h).

Table 10 .
Comparison of adsorption capacity and equilibrium time of various adsorbents for Hg 2+